1. Field of the Invention
The present invention relates to an optical pickup apparatus and an information recording and/or reproducing apparatus for performing information recording to an information recording medium and/or performing information reproduction from the recording medium.
2. Description of the Related Art
An optical disc such as a CD (Compact Disc) or a DVD (Digital Video Disc or Digital Versatile Disc) is well known as an information recording medium to/from which information recording or information reproduction is optically performed. Development has progressed for various optical discs including a read-only optical disc, a write-once optical disc on which additional recording can be performed and a rewritable optical disc in which erasing and re-recording of information data can be performed. In particular, development is being pursued concerning a high-density optical disc which has a large recording capacity.
Research and development of an optical pickup apparatus and an information recording and/or reproducing apparatus have been being progressed for performing the information recording and/or information reproduction to/from the above-mentioned optical discs. There is considered a method of coping with the high-density discs by increasing a numerical aperture (NA) of an objective lens provided in the pickup apparatus.
The optical disc has a structure such that a laser beam is irradiated through a light transmitting layer covering a recording surface. The thickness of the light transmitting layer, however, is not always uniform over the entire surface of the optical disc. If the numerical aperture NA of the objective lens is increased and, thus, a range of an incident angle of the laser beam is increased, the laser beam is greatly influenced by a spherical aberration due to a thickness variation or error of the light transmitting layer.
The thicknesses of light transmitting layers between, for example, a CD and a DVD are different. Therefore, the thickness difference exerts an influence similar to that by the thickness variation. Particularly, it is difficult to realize an optical pickup apparatus having compatibility when the numerical aperture NA of the objective lens is increased.
In order to solve the problem, there has been proposed an optical pickup apparatus such that a thickness variation of a light transmitting layer is optically detected from a returning light returned from a recording surface of an optical disc by a reflection or the like and a spherical aberration is compensated on the basis of a detection result (see, Japanese Patent Application Kokai No.2000-182254).
The pickup apparatus has a configuration shown in FIG. 1. For example, a laser beam is emitted from a light source 1 upon information reproduction. The laser beam is transmitted through a collimator lens 2, a beam splitter 3, a ¼ wavelength plate 4, and an objective lens 5, and is converted into a beam of a small irradiation diameter, and irradiated to a recording surface through the light transmitting layer of an optical disc 8.
The laser beam is reflected by the recording surface and the resultant returning light is transmitted through the objective lens 5, the ¼ wavelength plate 4, the beam splitter 3, and a condenser lens 6 and is detected by a photodetector 7. The laser beam is reflected and diffracted by the recording surface when the laser beam is irradiated on the recording surface of the optical disc 8 through the objective lens 5 as shown in FIG. 2, a 0-th order light RMB(0) and a diffracted light obtained by reflection and diffraction are transmitted as returning light through the objective lens 5, and the photodetector 7 detects the light intensity of the returning light.
The photodetector 7 separately detects a light intensity of each of a light RMBi passing through an inner radius portion of the objective lens 5 (hereinafter, referred to as an inner radius light) and a light RMBo passing through an outer radius portion of the objective lens 5 (hereinafter, referred to as an outer radius light) which are obtained by separating the returning light (the 0-th order light RMB(0) and the diffracted light) and generates a detection signal FE1 indicating the light intensity of the inner radius light RMBi and a detection signal FE2 indicating the light intensity of the outer radius light RMBo.
As shown in FIG. 3, the detection signal FE1 which is generated from the photodetector 7 is set to be a focusing error FE. The objective lens 5 is adjusted so as to be in an in-focus state within a capture range so that the focusing error FE is equal to 0. Further, the detection signal FE2 which is obtained when the objective lens 5 is in the in-focus state and the detection signal FE1 are compared by a differential amplifier, thereby detecting a thickness error SPHE of the light transmitting layer. The thickness error SPHE is set to a spherical aberration error. A position of the collimator lens 2 in the direction of an optical axis OA is finely adjusted so as to set the spherical aberration error SPHE to be 0, thereby compensating an influence of the spherical aberration which is caused by the thickness error of the light transmitting layer.
The conventional optical pickup apparatus, however, has the following problem.
As shown in FIG. 2, where the laser beam is irradiated to the recording surface of the optical disc 8, a diffraction pattern in a state where the 0-th order light RMB(0), a +1 primary diffracted light RSB(+1), and a −1 primary diffracted light RSB(−1) are partially overlapped is generated as shown in, for example, FIGS. 4, 5, and 6 in accordance with optical parameters such as numerical aperture (NA) of the objective lens 5 and a track pitch TP of the optical disc 8. A phenomenon such that each light intensity of overlapped portions X and Y in the 0-th order light RMB(0) is decreased or increased in dependence on an interference of the light occurs.
According to the conventional optical pickup apparatus, the focusing error FE is detected from the inner radius light RMBI, a focusing servo is performed, the focusing error FE is set to a reference, and the spherical aberration error SPHE is detected from the outer radius light RMBo, thereby compensating the influence of the spherical aberration. It is, therefore, necessary that the focusing error FE and spherical aberration error SPHE should be detected with high linearity and high precision.
When each light intensity of the overlapped portions X and Y in the 0-th order light RMB(0) is largely decreased or increased in dependence on the interference of the diffracted light, the focusing error FE and spherical aberration error SPHE cannot be detected with high linearity from the light RMBi(0) in the inner radius portion and the light RMBo(0) in the outer radius portion of the 0-th order light RMB(0). It causes a problem such that it is difficult to perform the focusing servo and spherical aberration compensation with high precision.
For example, as shown in FIG. 4, when the numerical aperture NA of the objective lens 5 is relatively small and the objective lens 5 is in the in-focus state or in the case where the laser beam is irradiated to an optical disc (for example, CD or the like) in which the track pitch TP is relatively narrower than the wavelength λ of the laser beam and the objective lens 5 is in the in-focus state, each interval L1 between each of the +1 primary diffracted light RSB(+1) and −1 primary diffracted light RSB(−1) and the 0-th order light RMB(0) increases, so that the areas of the overlapped portions X and Y in the 0-th order light RMB(0) relatively decrease.
Further, in a case shown in FIG. 4, if the objective lens 5 is in a defocusing state when information recording or information reproduction is actually performed, the amplitudes on the plus side and the minus side are almost equal and the focusing error FE of high linearity in an S-character curved shape is detected in accordance with a defocusing amount as shown in FIG. 7.
In case of FIG. 4, therefore, the linearity of each of the focusing error FE and the spherical aberration error SPHE is not deteriorated since the areas of the overlapped portions X and Y in the 0-th order light RMB(0) are relatively small, so that a problem such that the precision in each of the focusing servo and the spherical aberration compensation deteriorates does not occur.
When the numerical aperture NA of the objective lens 5 is relatively large and the objective lens 5 is in the in-focus state or in a case where the laser beam is irradiated to an optical disc (for example, land/groove recording disc or the like) in which the track pitch TP is relatively wider than the wavelength λ of the laser beam and the objective lens 5 is in the in-focus state, however, each interval L2 between each of the +1 primary diffracted light RSB(+1) and −1 primary diffracted light RSB(−1) and the 0-th order light RMB(0) decreases, so that the areas of the overlapped portions X and Y in the 0-th order light RMB(0) relatively increase. The light intensity of each of the overlapped portions X and Y in the 0-th order light RMB(0) is, therefore, greatly decreased or increased by the light interference as compared with the diffraction pattern shown in FIG. 4. Thus, a problem such that the detection precision of the focusing error FE and the spherical aberration error SPHE is deteriorated occurs.
Further, as shown in FIG. 6, when the numerical aperture NA of the objective lens 5 is relatively large or in a case where the laser beam is irradiated to an optical disc in which the track pitch TP is relatively wider than the wavelength λ of the laser beam and the objective lens 5 enters the defocusing state when information recording or information reproduction is actually performed, the light intensity of each of the overlapped portions X and Y in the 0-th order light RMB(0) is largely decreased or increased by the light interference. For example, therefore, as shown in FIG. 8, the focusing error FE in what is called a distorted S-character curved shape in which amplitudes on the plus side and minus side are different is detected.
As shown in FIGS. 5 and 6, therefore, the focusing error FE having a high linearity cannot be detected and a capture range is narrow and asymmetrical as shown in FIG. 8 when the numerical aperture NA of the objective lens 5 is relatively large or in the case where the track pitch TP of the optical disc is relatively wide. There is, consequently, a problem such that it is difficult to perform the focusing servo of high precision and thus to execute the spherical aberration compensation of high precision.
Particularly, it is extremely difficult to detect the focusing error FE having a high linearity when a light intensity of each of overlapped portions XI′ and YI′ of the inner radius light RMBi and the ±1 primary diffracted light RSB(+1) and RSB(−1) shown in FIG. 6 largely decreases, so that the focusing servo does not function and it is difficult to perform the spherical aberration compensation.
As mentioned above, the areas of the overlapped portions X and Y of the +1 primary diffracted light RSB(+1) and −1 primary diffracted light RSB(−1) to the 0-th order light RMB(0) increase when the numerical aperture NA of the objective lens 5 is increased so as to cope with the high density optical disc. Therefore, the focusing error FE and the spherical aberration error SPHE of high precision cannot be detected and thus a problem occurs that it is difficult to cope with the high-density optical disc.
Since the overlapped portions X and Y of the 0-th order light RMB(0) and the +1 primary diffracted light RSB(+1) and −1 primary diffracted light RSB(−1) occur by the diffraction irrespective of the value of the numerical aperture NA of the objective lens 5 or the value of the track pitch TP of the optical disc 8, it is an important object to suppress the decrease or increase of the light intensity in the overlapped portions X and Y in order to realize the optical pickup apparatus having compatibility and capability for the high-density discs.